The invention relates to a photoreceptor for positive electrostatic charge such as an electrophotographic photoreceptor for positive electrostatic charge.
Heretofore, as for the electrophotographic photoreceptors, there have so far popularly been used an inorganic photoreceptor bearing thereon a photosensitive layer mainly comprising such an inorganic photoconductive substance as selenium, zinc oxide, cadmium sulfide and the like.
On the other hand, in the recent years, it has been positively developed and researched to utilize a variety of organic photoconductive substance for the materials of the photosensitive layers of such electrophotographic photoreceptors.
For example, Japanese Patent Examined Publication No. 10496/1975 describes organic photoreceptors bearing thereon a photosensitive layer containing poly-N-vinyl carbazole and 2,4,7-trinitro-9-fluorenone. However, this photoreceptor is not always satisfactory because of its sensitivity and durability. With the purpose of improving such faults, there are attempts to develop organic photoreceptors having a high sensitivity and an increased durability in such a manner that, in a photosensitive layer, a carrier generating function and a carrier transport function are alloted separately to the different substances. In the so-called function separation type electrophotographic photoreceptors such as mentioned above, the substances capable of displaying each of the function may be selected from a wide range of substances. It is, therefore, relatively easy to make an electrophotographic photoreceptor having any desired characteristics.
Many substances have so far been proposed to serve as the carrier generating substances capable of being effectively used in such function separation type electrophotographic photoreceptors. Examples of those using an inorganic substance include amorphous selenium, as described in Japanese Patent Examined Publication No. 16198/1968, which is used in combination with an organic carrier transporting substance.
There has also been proposed many electrophotographic photoreceptors each having used an organic dyestuff or an organic pigment to serve as the carrier generating substances thereof. They include, for example, those having a photosensitive layer containing a bisazo compound, which have already known in Japanese Patent Publication Open to Public Inspection Nos. 37543/1972, 22834/1980, 79632/1979, 116040/1981 and the like.
Well-known photoreceptors using an organic photoconductive substance are normally used for negative electrostatic charge. The reason thereof is that they are advantageous in photosensitivity and the like because the hole mobility of carriers is great when an electrostatic charge is negative.
In the use of such a negative charge, it is verified that there are the following problems. Namely, taking precedence of all other problems, there is such a problem first that ozone is apt to produce in atmosphere when a negative charge is applied by an electric charge, so that the environmental conditions may be worsened. One of the other problelms is that toners of positive polarity are required to use in the development of a photoreceptor for negative electrostatic charge and the toners of positive polarity can hardly be prepared from the viewpoint of triboelectrification series to ferromagnetic carrier particles.
It is, accordingly, proposed to use a photoreceptor using an organic photoconductive substance with a positive charge. For example, in the case of a photoreceptor for positive electrostatic charge in which a carrier transport layer is laminated on a carrier generation layer and the carrier transport layer is formed of a substance having a relatively great electron transport function, the carrier transport layer has to contain trinitrofluorenone or the like which is, however, not suitable for use because this substance is carcinogenic. On the other hand, it may be possible to use a photoreceptor for positive electrostatic charge, which is prepared by laminating a carrier generation layer on a carrier transport layer having a relatively great hole transport function. With this photoreceptor, however, there is an extremely thin carrier generation layer on the surface of the photoreceptor, therefore the printing resistance and the like are deteriorated and the layer arrangement thereof is not suitable for practical use.
U.S. Pat. No. 3,615,414 indicates a photoreceptor for positive charge containing a thiapyrylium salt, i.e., the carrier generating substance, so as to form an eutectic complex with polycarbonate, i.e., the binder resin. In the well-known photoreceptor, there are the faults that a memory phenomenon is relatively serious and ghosts are apt to cause. U.S. Pat. No. 3,357,989 also indicates a photoreceptor containing phthalocyanine. However, phthalocyanine is varied in its characteristics because of the crystal systems and besides the crystal systems are to be strictly controlled, and further not only the sensitivity in short wavelength is not enough but also the memory phenomenon is serious, therefore, such a photoreceptor is not suitably used with a copying apparatus using a light source having a wavelength region of visible light.
As it has so far been, the photoreceptors using an organic photoconductive substance have been scarcely ever feasible of using for positive electrostatic charge, therefore they have so far been used solely for negative charge.